1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display capable of preventing deformation of optical sheets by heat emitted from a light source.
2. Description of the Related Art
In general, a liquid crystal display (hereinafter referred to as “LCD”) displays desired pictures by controlling the amount of transmission of light supplied from a backlight unit to a liquid crystal panel. The liquid crystal panel comprises a plurality of liquid crystal cells arranged in a matrix form and a plurality of control switches for switching a video signal supplied to each of the liquid crystal cells.
FIG. 1 is a perspective view illustrating a liquid crystal display of the related art.
Referring to FIG. 1, the liquid crystal display of the related art has a backlight unit containing a light source 22, a light source housing 4, a light guide panel 2, a plurality of optical sheets 8, 10 and 12, and a reflection plate 6. The light source 22 generates light. The light source 22 is installed in a light source housing 4. The light guide panel 2 converts the light incident from the light source 22 into a planar light source. The plurality of optical sheets 8, 10 and 12 are attached on the light guide panel 2 for improving the light efficiency incident on a display panel. The reflection plate 6 is attached to the rear surface of the light guide panel 2 for reflecting the light emitted towards a rear surface of the light guide 2 back towards the liquid crystal panel 20. The liquid crystal panel 20 provides images using the light from the light source 22.
In the related art, the light source 22 is typically a cold cathode fluorescent light source, and the light generated from the light source 22 is incident to the light guide panel 2 through the incident surface formed in a side of the light guide panel 2.
The light source housing 4 is installed to enclose the light source 22 and has a reflection plate in its inside to reflect the light from the light source 22 to the incident surface of the light guide panel 2.
The light guide panel 2 converts the light incident from the light source 22 into the planar light source. The light guide panel 2 has a slanted rear surface and a flat front surface, which is perpendicular with respect to the incident surface of the light guide 2. Installed in the rear surface of the light guide panel 2 is a reflection plate 6 facing thereto. The light guide panel 2 takes the light incident from the light source 22 and directs it to a point that is distant from the light source 22. The light guide panel 2 generally has a high strength and thus is not easily deformed or fractured. Conventional light guide panels are formed with a polymethylmethacrylate PMMA having a high transmittance.
Light that is incident on the reflection plate 6 from the rear surface of the light guide panel 2 is reflected back to the light guide panel 2, thereby increasing the amount of light directed towards the liquid crystal and reducing the light loss. If the light from the light source 22 is incident to the light guide panel 2, the light is reflected by a predetermined slanting angle in the slanted lower surface such that it is uniformly directed to the exit surface. The light proceeding to the rear surface and the side of the light guide panel 2 is reflected on the reflection plate 6 to proceed to the front surface. A diffusion sheet 8 spreads the light that exits via the front surface of the light guide panel 2 over the entire surface of the diffusion sheet 8.
The light efficiency increases when light incident to the liquid crystal panel 20 is perpendicularly incident to the liquid crystal panel 20. To this end, two prism sheets 10 and 12 are stacked on the diffusion sheet 8 so that the angle of the light that exits from the light guide panel 2 becomes perpendicular to the liquid crystal panel 20. The first and the second prism sheets 10 and 12 contain a plurality of prism bars each having a ridge and a groove. The prism sheets 10 and 12 serve to collect the light that has exited from the diffusion sheet 8 and provide it to the screen in a direction perpendicular to the screen.
A protection film (not shown) may be formed on the second prism sheet 12. The protection film protects the surface of the second prism sheet 12 helps to distribute the light uniformly.
The light generated from the backlight unit is incident on the liquid crystal panel 20.
The liquid crystal panel 20 includes liquid crystal cells (not shown) which are disposed in an active matrix type between an upper array substrate 20a and a lower array substrate 20b. Each of the liquid crystal cells includes a thin film transistor for switching a video signal installed therein. A refraction ratio of each of the liquid crystal cells is changed in accordance with the video signal, whereby the pictures corresponding to the video signal are displayed. That is, the liquid crystal is driven by a voltage difference of the pixel electrode included in the lower array substrate 20b and a common electrode included in the upper array substrate 20a to selectively transmit the light from the backlight unit and thereby display the pictures.
When power is supplied to the liquid crystal display of the related art after the liquid crystal display has been inactive for a long time, warping occurs in the region(s) of at least one of the optical sheets 8, 10, 12 (herein referred to as the optical sheet 16) most proximate to the light source 22. As shown in FIG. 2, the reason is that warping occurs in the various sheets, as typified by the optical sheet 16 illustrated in the figure, due to the large amount of heat generated by the light source 22. More specifically I in the optical sheet 16, a relatively large temperature difference arises between a region adjacent to the light source 22 and the display region of the optical sheets 16 as well as a region of the optical sheet 16 relatively distant from the light source 22. This, in turn, causes non-uniform expansion of the optical sheet 16, which induces the warping in the optical sheet 16, especially the side of the optical sheet 16 most proximate to the light source 22 and contributes to non-uniformity of display.